The olfactory system discriminates thousands of odorants with very high sensitivity. Olfactory receptors expressed in olfactory epithelium play a crucial role for the recognition specificity. Little is known about the recognition mechanisms of olfactory receptors since so far only one cloned olfactory receptor gene (odr-10) in C. elegans has known specific odorant (diacetyl). However, in goldfish, 17alpha, 20beta-dihydroxy-4- pregnen-3-one and prostaglandin F are extremely potent pheromones. Nanomolar levels of these pheromones can elicit electrical responses. These pheromones critically regulate the reproductive behavior of fish. Evidence also suggest that fish have fewer olfactory receptors than do mammals, making it a simpler system for studying olfaction. I propose to study the specificity of pheromone recognition in goldfish. By combining calcium imaging experiments, single cell cDNA library construction, and differential screening, I will clone these pheromone receptors. Expression and characterization of the receptors will be carried out to elucidate the underlying pheromone recognition mechanism. My hope is that these experiments will provide new insights into the combinatorial recognition strategy that is used in olfaction.